In recent years, attention has been increasingly focused on large screen displays such as home theaters and presentations, and projectors are recently being commercialized which use a small reflective light valve of a liquid crystal on silicon (hereinafter, referred to as LCOS) in which a switching element, a reflection electrode or the like is formed on a silicon substrate or a digital micromirror device (hereinafter, referred to as DMD), and enlargedly project a display image with a projection lens to obtain a large screen display image.
The LCOS is one of the SLMs, and it has reflection pixels in a form of matrix, and can switch displays at a high speed using a video signal. In order to display moving pictures at a video rate, it is necessary that video of 60 frames can be displayed within one field. For that purpose, the liquid crystal response speed of at least 1/60=16.7 msec or lower is required. Further, in order to display at least three colors (RGB) during that time, a response speed of 5.6 msec is required. As examples of such a high-speed response liquid crystal, there are a ferroelectric liquid crystal, an antiferroelectric liquid crystals, an OCB (optically Compensated Bend) liquid crystal and the like. In the OCB liquid crystal, a Bend orientation cell is used to self-compensate changes in the visibility angle direction using birefringence of the liquid crystal, and when this liquid crystal is combined with a negative optical compensation film, a wider visibility angle is realized, as well as a high-speed response is enabled.
The DMD is one of the SLMs, and is mainly used as a projection-type display. The DMD has hundreds of thousands or one million or more extremely minute mirrors on one chip, each of the mirrors corresponding to one pixel. ON/OFF of the DMD is controlled by inclining these mirrors to change the reflection angles of beams which are incident on the mirrors. For that purpose, the respective mirrors are mounted to one or more hinges which are mounted on a supporting post, and are separated from a control circuit situated below by an air gap. This control circuit applies static electricity, which selectively inclines the respective mirrors. When this is applied to a display, image data are loaded on a memory cell of the DMD, and the mirrors are inclined on the basis of these data to reflect light toward the ON direction or away from the ON direction.
As methods adopted in the projectors, when classified according to the number of SLMs required in the projector, a single-panel type and a three-panel type are mainly used. As an example of the three-panel-type projectors, there is one which modulates light beams of respective colors, which has been subjected to the color separation, by the corresponding SLMs, respectively, and then performs the color composition while projecting the light on a screen. In this method, three SLMs are used in parallel, the respective being used for red (R), green (G), and blue (B). On the other hand, in the single-panel-type projector, only one SLM is used, and R, G and B light beams are modulated successively in a time-multiplexing manner, or spatially in units of area or pixel, while using a single-panel SLM. Accordingly, in the single-panel-type projector which requires only one SLM, requests to hardware relating to the SLM are only one-third of those in the three-panel-type projector which requires three SLMs. This is not restricted to the projectors, but applies to all color display devices using the SLMs.
Hereinafter, the color display device using the single-panel projector is described.
As an example of the color display device using the single-panel projector, there is a time-multiplexing color sequential type color display device utilizing a time-multiplexing color mixture. In this time-multiplexing color sequential method, the pixels have red, green and blue values, respectively, and during each frame period, the pixels in the frame are addressed successively according to red, blue, and then green data. On the other hand, filters of the same colors as these colors are positioned in the form of a disk, a color wheel having at least three different color regions is synchronized with these data, and data corresponding to the respective colors are displayed by the SLM. At this time, the band of light incident on the SLM is controlled by the color wheel. As described above, the time-multiplexing field sequential color display device enables color display in a time-multiplexing manner and, when the time-multiplexing rate is higher than the standard display speed of 60 images/sec, the images are perceived by the eyes to have original colors.
The above-mentioned prior art field sequential color display device using the color filter is described with reference to FIG. 36. FIG. 36 is a diagram schematically illustrating an example of the prior art field sequential color display device using the color wheel. As shown in FIG. 36, the field sequential color display device comprises a lamp 1001, an ellipsoidal mirror 1002, an UV-IR cut-off filter 1003, a color wheel 1004, a condensing lens 1005, a field lens 1006, a reflective LCOS 1007, and a projection lens 1008.
The lamp 1001 is a discharging-type high output lamp such as a xenon lamp, a metal halide lamp, and an extra-high pressure mercury lamp.
The reflective LCOS 1007 is one of the SLM.
The color wheel 1004 is preferably situated in a position where beams are condensed the most. This is because the SLM should be turned off to prevent color mixture, while the color wheel is being rotated and a beam spot is passing through the boundary of the different color filters, and the shorter the OFF time is, the higher the temporal opening ratio is, whereby brighter displays are enabled. Therefore, it is preferable that the condensation spot on the color filter should be smaller to miniaturize the color wheel, otherwise a color wheel having a larger outer diameter is required, resulting in a considerably large size of the entire system.
The operation of the so-constructed prior art field sequential color display device is described. The lamp 1001 is positioned approximately in a focus position of the ellipsoidal mirror 1002 as a concave mirror, so that the emitted white light beams are condensed by the ellipsoidal mirror 1002 on the color filter of the color wheel 1004. The UV-IR cut-off filter 1003 filters out ultraviolet and infrared rays of the light emitted from the lamp 1001. The color wheel 1004 comprises red, blue, and green color filters which are positioned in the form of a disk and, in synchronization with the filtering of beams by the respective color filters, the LCOS 1007 displays image frames of the beam color. Normally, the color wheel 1004 is rotated one revolution per image frame in 1/60 sec, or at 3600 rpm. The condensing lens 1005 efficiently condenses light which is transmitted through the color wheel 1004, and irradiates the LCOS 1007. The field lens 1006 is used for condensing light which is transmitted through the LCOS 1007 on the projection lens 1008.
In this prior art field sequential color display device, there are at least three color sub-frames during one frame frequency, the sub-frames being red, green and blue, respectively. The LCOS 1007 switches display images at a considerably high speed for the respective colors, and modulated beams of respective colors are enlargedly projected on a screen (not shown) by using the projection lens 1008. Since videos of the respective colors (R, G and B) are successively projected and displayed on the screen in 1/60 sec, these videos are perceived by the eyes as after-images, whereby full-color videos are recognized.
In the above-mentioned prior art time-multiplexing color sequential type color display device, the color wheel is rotated by a motor or the like at a high speed. Therefore, it is quite important how the rotation speed and phase of the color wheel are controlled, to accurately and precisely acquire timing information for switching the colors of red, green and blue, and further control the SLM to perform modulation in synchronization with the color.
Accordingly, in the prior art field sequential color display device, a reflective photo-sensor has been commonly used for detecting the position of the color wheel. FIG. 37 is a schematic diagram illustrating a color wheel, and a cross-sectional view illustrating a color wheel assembly which is constituted by a color wheel and a motor. A hub 372 of the color wheel 1004 is painted black in its entirety, and an aluminum tape 373 is pasted as an index mark at a position of the joint part of a green filter 1004G and a red filter 1004R. The reflective photo-sensor 374 is mounted on a case 375 which houses the color wheel 1004, and when the color wheel 1004 is rotated, the reflective photo-sensor 374 detects the aluminum tape as a reflecting surface and generates a pulse signal of one pulse per one revolution. Thereby, the control circuit of the SLM performs the switching from a green video drive signal to a red video drive signal, as well as controls the rotation speed and phase of the motor so that the color wheel 1004 is rotated at one frame frequency. An example of the method for receiving a pulse feedback from the color wheel and controlling the rotation speed and phase of the motor is described in detail in U.S. Pat. No. 5,868,482.
In the above-mentioned prior art field sequential color display device using the color wheel, when a desired display quality is to be obtained without color separation, the number of revolutions of the color wheel 1004 should be about 10000 rpm or larger. However, in this high-speed rotation, the centrifugal force applied to the color wheel 1004 becomes quite large, whereby the aluminum tape 373 pasted on the color wheel 1004 as the index mark is soon peeled off and flew into pieces.
In addition, since the color wheel 1004 is positioned in close proximity to the lamp 1001 as well as the beams which have condensed in a small spot on the color wheel 1004 are subjected to the color separation, the color wheel 1004 is easily affected by the heat and its temperature immediately rises at 70° C. or more. Accordingly, the adhesive of the aluminum tape 373 bonded on the color wheel 1004 as the index mark has a poor adhesion as compared to room temperatures. Therefore, the tape 373 becomes more easily peeled off. Further, when the color wheel is housed in the case, the temperature of the color wheel case itself is increased due to heat radiated from the lamp or absorption of unnecessary light, whereby it becomes difficult to cool the color wheel and the motor in the case.
Further, in the manufacture of the color wheel, steps of painting the hub 372 in black, and positioning and bonding the aluminum tape 373 as the index mark on the hub are required. Further, the color wheel is housed in the case to be protected from the dust, and therefore, a step of installing the photo-sensor 374 for detecting the index mark formed on the color wheel 1004, at a predetermined position of the color wheel case 375 is required. These steps both should be carried out accurately, which leads to increases in costs.
Further, a motor 371 is mounted at an opening below the flange of the color wheel 1004, and the color wheel 1004 is rotated by the motor in the case 375. At this time, the photo-sensor 374 for detecting the index mark is mounted on the case 375 so as to protrude toward the color wheel 1004. As shown in FIG. 37, the color wheel 1004 opposes a bottom surface 375a and a case lid 375b of the color wheel case body 375. The color wheel 1004 is in proximity the bottom surface of the color wheel case body 375.
When the color wheel 1004 is rotated, the circumferential speeds are different between in the vicinity of the rotation axis and the outer circumference part. Therefore, an air current from the center of the color wheel 1004 toward the outer radius occurs in a gap between the color wheel 1004 and the case body 375 (shown by arrows in FIG. 37). At this time, the photo-sensor 374 interferes with the air current, leading to noises.
Further, to allow the photo-sensor 374 to read the index mark, the hub area through which light does not directly pass is required, and this presents a problem in minimizing the diameter of the color wheel or miniaturizing.
The color wheel rotates color filters which are made of glass at a high speed, so it is easily electrostatically charged due to friction with air. when the color wheel is charged, it attracts dust in the air, thereby reducing the transmittance of the filter. Even when the color wheel is housed in the case, since the rotation of the color wheel creates wind pressure, and air frequently flows into or out of the gap of the case, the filters similarly become dirty with time. Especially when the color wheel is housed in the case, it is necessary to provide an opening for incoming or outgoing light, and the incoming/outgoing air into/from the opening causes the dust to be caught in the case.
When the color wheel which is constituted by thin glass filters rotates at a high speed and cuts through the air, a whistling sounds occur, and it becomes the source of large noise, together with the electromagnetic sounds of the motor. Especially when the color wheel is housed in the case, the air current is generated by the wind pressure resulting from the rotation of the color wheel from an opening which is provided for incoming or outgoing light, and the incoming/outgoing air into/from the opening causes noise.
In the field sequential color display device as shown in FIG. 36, a condensation spot 1009 of light emitted from the lamp 1001 is formed on the color wheel 1004. The size of the condensation spot 1009 depends on the size of an emitting part 1100a of the lamp 1001, and the larger the emitting part 1001a is, the larger the condensation spot 1009 is.
FIG. 38 is a diagram for explaining the relationship between the color wheel 1004 and the condensation spot 1009. Hereinafter, the problems of the prior art field sequential color display device are described with reference to FIG. 38.
The color wheel 1004 comprises, for example, red, green and blue fan-shaped color filters 1004R, 1004G and 1004E which are combined in the form of a disk, and a full-color display is enabled by rotating the color wheel in synchronization with the display of the LOOS 1007. However, when the condensation spot 1009 extends across two adjacent color filters, light beams which have been transmitted through the two color filters are incident on the LCOS simultaneously, resulting in a mixture of colors, whereby an image having a different color from the one which is to be normally displayed is displayed on the screen.
Practically, while boundaries 1004RG, 1004GB and 1004BR of the respective color filters 1004R, 1004G and 1004E are passing through the condensation spot 1009, the LCOS 1007 is controlled to display black, i.e., in the OFF state, whereby the above-mentioned problem of color mixture is solved. (Hereinafter, the period during which the LCOS 1007 is controlled to display black is referred to as a black display period.)
However, it is known that the light source used in the field sequential color display device, such as the lamp 1001, has the emitting part 1001a whose size (hereinafter, referred to as an arc length) varies during use. Usually, the arc length tends to be longer with the lighting time of the lamp 1001. Therefore, while the lamp 1001 is being used, the size of the condensation spot 1009 on the color wheel 1004 is gradually increased and, in some cases, the period during which the condensation spot 1009 extends across two adjacent color filters becomes longer than the black display period of the LCOS 1007. In these cases, the initially set black display period cannot prevent the formation of a color-mixed optical image on the LCOS 1007, whereby an image having a different color from the one which is to be normally displayed is displayed on the screen.
Assuming that the size of the condensation spot 1009 which is formed on the color wheel 1004 is gradually increased with changes in the arc length of the lamp 1001, the black display period of the LCOS 1007 can be set to be longer. However, the longer the black display period is, the more the ratio of light which irradiates the LCOS 1007 and contributes to the original image display is reduced. Therefore, in an initial stage of use when the arc length of the lamp 1001 is relatively short, an unnecessary black display period is set, whereby the light utilization efficiency is reduced and the luminance of the image projected on the screen is reduced.